This invention relates to a valve seat insert and more particularly to an improved insert for forming the valve seat for an internal combustion engine.
In conjunction with internal combustion engines, it is the practice to employ light alloy casting for the cylinder head. In order to permit more wear resistant, longer life operation, it has been the practice to provide an annular insert at the termination of the gas flow ports which serves as the seating surface for the popper valve that controls the flow through the gas port. It is extremely important that the insert piece be well retained in the cylinder head for obvious reasons. It is generally the common practice to press fit the valve seat into the cylinder head. Although such press fitting operations normally provide good initial attachment, certain problems can occur during running of the engine, particularly as a result of the thermal stresses due to the differences in degrees in thermal expansion between the cylinder head and the valve seat insert and also as a result of the initial stresses in the cylinder head and insert caused during installation.
Where the engine is provided with multiple valves the amount of cylinder head material between adjacent valve seats may be extremely small and this gives rise to a problem of cracking. In addition, the bond between the cylinder head material and the valve seat can also become damaged either on installation or during running operation.
It has been discovered that one problem attendant to the previous valve inserts and methods of installation has been that the pressure applied to the insert when it is pressed into place can cause forces to be exerted on the insert which result in tensile stresses in the insert. Since the insert material is normally stronger in compression than in tension, these tensile stresses can cause failures either at installation or failures which do not manifest themselves until after the engine has run for some time period.
It is, therefore, a principle object of this invention to provide an improved valve seat and method of inserting the valve seat wherein tensile stresses on the insert during installation are substantially eliminated.
In the previous proposed methods for inserting valve seats it has been also noted that during the installation phase due to the way in which forces are applied and the shape of the insert and the receiving recess that the insert tends to have its diameter enlarged upon installation at least in localized areas.
In order to further ensure good retention of the valve seat insert into the cylinder head, particularly where multiple valves are employed, it has been proposed to weld the insert to the cylinder head. This is done normally by a resistance welding technique wherein the insert is pressed into position while electrical current is applied to it so as to effect a weld between the insert and the cylinder head material. Resistance welding in this manner thus has many similarities to the use of pressed in inserts and can present the same potential damage for the reasons as aforenoted.
As has already been noted, the valve seat insert is formed from a different material from the main cylinder head material and the resistance welding of these dissimilar materials, particularly in the application for valve seats can give rise to additional problems. The different thermal expansions between the insert and the cylinder head can give rise to stresses between the insert and the cylinder head material even when welded in position.
It is, therefore, a principal object of this invention to provide an improved valve seat insert that will permit the formation of valve seats for internal combustion engines that will not be dislodged and which can be conveniently and effectively welded into the cylinder head.
It is a further object of this invention to provide a valve seat insert for an engine cylinder head having a composition that will lend itself to good bonding with the cylinder head material on insertion.
It is a further object of this invention to provide an improved valve seat insert for forming a cylinder head construction having a cylinder head and valve seat formed from different materials but providing an immediate boundary layer which is comprised of at least an alloy between these two materials and one of progressively different chemical composition between the base insert material and the base cylinder head material.